1. Field of the Invention
The present invention relates in general to thermal sensing devices that employ pyroelectric materials and more specifically to sensing device constructions that are designed
TO IGNORE SPURIOUS SIGNALS. 2. Description of the Prior Art
All known polymeric materials possessing pyroelectric properties also possess piezoelectric properties. A variety of thermal sensing devices employing such polymer pyroelectric/piezoelectric sensing materials are taught in the art. It is also taught that such sensing devices are highly sensitive to extraneous signals (noise) due to vibration of the sensing material. In at least one prior art publication, a sensing device is described as being constructed to provide a housing in which a pyroelectric polymeric film of small area is disposed to insulate the sensing medium from acoustical vibrations imposed on the pyroelectric film. "Intrusion-Detection Devices and Systems for air Base Security," Stanford Research Institute (1973).
Although the above prior art sensor is an improvement over detectors having sensing mediums exposed to surrounding conditions it still is deficient in that it is sensitive to forces that cause the entire housing to vibrate and it does not provide solid reliable connections between sensing circuitry employed in the detector and the sensing medium. Moreover, such sensor is not designed for utilizing a large area sensing medium such as employed in the present invention and in fact the author of the publication indicates at page 54 that future sensors should be limited to using sensing mediums with an area of only 1 cm.sup.2. We have found that sensors employing large area sensing mediums are vulnerable to not only compressional stress, but also to stresses that cause bending of the sensing medium. No known sensor device prior art has recognized or dealt with avoiding both compressive and bending stresses in sensors employing large area sensing mediums.
One prior art U.S. Pat., No. Rossin 3,839,640 discloses an intrusion detection device that includes a small area sensing medium attached by contact cement to a cube of plastic insulating foam in a construction that appears to be designed to prevent bending of the medium. However, the Rossin disclosure clearly points out that the cube is merely used to provide a framework for supporting the medium and in no way indicates that a similar type support could be used for a large sensing medium to prevent noise from being produced through bending of the medium. This is because Rossin was not confronted with the problem with which the present invention is designed to overcome because small area devices do not experience significant noise generation due to bending.